alleryA prescribed lens of power plus 2.5 D is a convex lens of focal length:
A20 cm
B50 cm
C40 cm
D30 cm
Answer & Solution
Correct answer: C. 40 cm
1. Focal length in metres is the reciprocal of the power.
2. One divided by 2.5 is 0.40 metre.
3. So it is a convex lens of focal length 40 cm.
_Source: syllabus Physics Part II, Ch 9 'Ray Optics and Optical Instruments'_
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